This invention relates to a tone signal generating device which generates a tone signal of a smooth waveform by waveform interpolation operation utilizing digital filter operation, and more particularly to controlling an interpolation characteristic in correspondence with a pitch of a tone to be generated.
Japanese Patent Laid-open Publication No. 63-168695, which corresponds to U.S. patent application Ser. No. 139, 659, proposes simplified filter operation realized by performing filter operation of m orders with respect to n tone signal sample data (where n&lt;m). More specifically, it discloses that while tone signal sample data are sequentially generated in correspondence with an integer part of an address signal, n filter coefficients are selected from among filter coefficients of m orders in correspondence with the value of a decimal part of the address signal (combination of orders of filter coefficients to be selected differs depending on the decimal part of the address signal), and then the selected n filter coefficients are arithmetically operated with n tone signal data so that filter operation of m orders may be virtually performed.
Also, in this Japanese Patent Laid-open Publication No. 63-168695, it is disclosed that data stored at n addresses in a waveform memory, rather than data generated during fixed n sampling periods, are used as the n tone signal sample data. This means that rather than simple filter operation in accordance with constant sampling periods, filter operation is performed whose sampling period changes with a pitch of a tone to be generated, so as to effect waveform interpolation operation for n samples. As the result of such interpolation operation, one sample data is produced.
It is already known that, in the case where it is intended to eliminate an aliasing noise with a digital filter, what is required is to set the filter characteristic to be a low-pass filter characteristic and also to set the cut-off frequency at a frequency lower than a half of the sampling frequency fs. Thus, also in the case of the above-mentioned waveform interpolation operation, elimination of an aliasing noise can be effected by making the filter characteristic a low-pass filter characteristic and setting the cut-off frequency at a frequency lower than a half of the sampling frequency fs.
However, if a digital filter is utilized to perform waveform interpolation operation as shown in the Japanese Patent Laid-open Publication No. 63-168695, a virtual sampling period will change as a tone pitch changes, so that a resultant filter characteristic will also undesirably shift (without the coefficient values being changed) as a tone pitch changes. This is due to the fact that, in the case of waveform interpolation operation utilizing a digital filter, a unit delay time in the digital filter does not become constant, but it changes as a pitch changes. This causes inconveniences when it is desired not to shift the filter characteristic in correspondence with a pitch.
For example, when waveform interpolation and elimination of an aliasing noise are to be simultaneously performed, with the filter characteristic for use as an interpolation characteristic being made a low-pass filter characteristic, the cut-off frequency for the desired low-pass filter characteristic will change in correspondence with a pitch. Thus, the inconveniences that an aliasing noise can not be eliminated occurs if the pitch becomes higher and the cut-off frequency becomes higher than the frequency of the aliasing noise. The inconveniences may be forestalled if arrangements are made such that waveforms of sufficiently high resolution are produced in advance so as not to introduce such inconveniences at the maximum pitch of a tone to be generated. In that case, however, there will not be much use for performing waveform interpolation.